How can i manually enter a number to be blocked

I have T-Mobile digits service and I keep getting a text i don’t want, when I checked T-Mobile’s support page about blocking numbers it says go to your devices call block settings. iOS doesn’t seem to have an option to manually enter a number to block, only numbers that have contacted you. Seeing as how the text came through T-Mobile’s digits app and not to the phone, I can’t block the number. Has anyone else had this problem and found a work around?